This invention relates to a method of corrosion rate detection used for diagnosing deterioration, attributable to corrosion, of civil engineering structural members of steel material, especially, those installed in a corrosive environment such as seawater or river water.
The useful service life of a structural member of steel material used in a corrosive environment, such as, that partly submerged in seawater or river water is necessarily limited due to formation of pits or reduction of its thickness by corrosion. On the other hand, it is desirable that such a structural member can be used for a longest possible period of time from the economical aspect, as long as it is not rendered dangerous as part of structure. Therefore, there is a growing demand for detecting the present status of corrosion of the structural member and diagnosing the degree of corrosion for the purpose of estimating its residual service life. It is indispensable for the diagnosis of the status of corrosion to accurately detect the rate of corrosion.
Various methods are now employed for the detection of the rate of corrosion of such a structural member. According to a first method, a small test piece for corrosion monitoring purpose is previously attached to an equipment or a structural member of steel material, and is periodically checked to find a decrease in the weight due to corrosion, and the value of the decreased weight is divided by the period of the test to detect the rate of corrosion of the equipment or structural member. According to a second method, for example, a method described in a paper entitled "Technical Note of the Port and Harbour Research Institute" No. 413, page 5 (1982), the plate thickness of a structural member of steel material is measured by an ultrasonic thickness gauge, and the difference between the remaining plate thickness and the initial plate thickness of the structural member is divided by the period of use to detect the rate of corrosion of the structural member. According to a third method, for example, a method described in JP-A-55-47433, published on Apr. 3, 1980 a pair of test pieces made of the same material as that of an object whose corrosion rate is to be measured are placed in an environment in which the object is installed, and the rate of corrosion of the object is detected on the basis of an electrochemically measured impedance between the test pieces.
However, data obtained by the first and third methods provide merely information of the rate of corrosion of small test pieces and do not reflect the actual rate of corrosion of an equipment or a structural member itself. Thus, such information is not indicative of the true rate of corrosion of the equipment or structural member itself. Although the second method is more practical than the first and third methods, the initial plate thickness of a structural member is not usually previously measured in most cases, and reference to the plate thickness is usually obtained from known data printed in a catalog. However, tolerance of the plate thickness is allowed in JIS A5525 (Japanese Industrial Standards), and the reference to the catalog data including the tolerance may lead to the cause of an error.
Further, when a paint or like layer is previously coated on the surface of a structural member of steel material for the purpose of temporarily preventing corrosion or when a structural member of steel material is considerably corroded, measurement of the plate thickness by an ultrasonic thickness gauge leads to an error which adversely affects the calculation of the rate of corrosion. Further, the third method is not necessarily efficient in that measurement over an entire available frequency range is required because the frequency response characteristic of the phenomenon of corrosion has not yet been clarified.